Fireworks involve many different concepts from chemistry. First the firework is rocketed into the sky using gunpowder. The gunpowder is made from a mixture of a fuel and an oxidizer. Fuels are molecules which contain a lot of energy in their chemical bonds, and oxidizers are molecules which help break those bonds to release the energy. You just need to provide a little activation energy (a spark or flame) and the chemical reaction will start.

When the firework gets into the sky, it explodes. The firework is filledwith small capsules called "Pyrotechnic Stars". These are the balls of light you see flying out of the firework when it explodes. These also contain fuel and an oxidizer, along with a metal powder. When the chemical reaction between the oxidizer and the fuel occurs, the energy from the fuel is released as heat and pressure (which causes the firework to fly apart).

The "Pyrotechnic Stars" also contain a metal powder. When you put metals in flames they change the color of the flame! For example, if you put copper powder in a flame it turns the flame blue-green. There are all sorts of different metals which can make the different colors you see in the fireworks.

On the molecular level, the reason for this is that the electrons in the metal atom become "excited," meaning they have more energy than normal.They want to get rid of this energy somehow, so they get rid of it by shooting out light. Some metals get excited more easily than others, so they shoot out light of different colors. For example, lithium is a metal which doesn't let its electrons get very excited so it shoots out red light, which carries less energy. But copper lets its electrons go crazy so it shoots out blue light, which has a lot of energy. Hope that answers your question!